Method of manufacturing ball joint

ABSTRACT

An intermediate molding ( 41 ) is formed so that a cylindrical arm-to-be portion ( 42 ) projects from the cylindrical outer surface of a body portion ( 13 ) of the arm-to-be portion ( 42 ). The arm-to-be portion ( 42 ) has a given thickness and a hole ( 42   a ) with a diameter greater than the inner diameter of a projecting connector portion ( 20 ) of an arm portion ( 15 ). A pair of flat face portions ( 42   b ) extending parallel to the axis of the body portion ( 13 ) are formed on the cylindrical outer surface of the arm-to-be portion ( 42 ). A tap ( 44 ) that has an external thread ( 43 ) formed along its cylindrical outer surface is inserted into the hole ( 42   a ) of the arm-to-be portion ( 42 ). Concave faces ( 49 ),( 49 ) are concaves respectively formed on the opposing surfaces of an upper die ( 47 ) and a lower die ( 48 ) so as to have a shape like a half cylinder with a curvature corresponding to that of the cylindrical outer surface of the projecting connector portion ( 20 ). The concave faces ( 49 ),( 49 ) sandwich and apply pressure to the arm-to-be portion ( 42 ), thereby narrowing the arm-to-be portion ( 42 ) and forming an internal thread ( 19 ) in the cylindrical outer surface of the hole ( 42   a ) of the arm-to-be portion ( 42 ). With the tap ( 44 ) being removed while being rotated, formation of the arm portion ( 15 ) is completed. Compared with a method that calls for forming an internal thread ( 19 ) by threading with an external thread ( 43 ) of the tap ( 44 ), the method of the invention enables the easier and quicker formation. The invention also prevents wear of the tap ( 44 ) and enables the easy formation of the projecting connector portion ( 20 ) of the arm portion ( 15 ) having stable characteristics and a desired internal thread ( 19 ).

TECHNICAL FIELD

The present invention relates to a method of producing a ball joint andits housing, wherein an internal thread is formed in the inner surfaceof the arm portion of the ball joint.

BACKGROUND OF THE INVENTION

An example of conventional ball joints is shown in FIG. 7.

The conventional ball joint 51 shown in FIG. 7 has a housing 56, inwhich a generally spherical ball head portion 53 is enveloped, with abearing seat (not shown) disposed between the spherical ball headportion 53 and the housing 56. The ball head portion 53 is provided atan end of a generally rod-shaped ball stud 52. The bearing seat isformed of synthetic resin. The housing 56 has a generally cylindricalbody portion 55. The body portion 55 is provided at one end thereof withan opening from which a stud portion 54 of the ball stud 52 isprojected. An arm portion 58 extends from the cylindrical outer surfaceof the body portion 55 of the housing 56 as an integral body with thebody portion 55. By screwing an external thread (not shown) in the armportion 58, another ball joint or the like can be coupled to the housing56. The arm portion 58 has a generally cylindrical shape in which aninternal thread 57 that permits an external thread to be screwed thereinis formed. The production process of the arm portion 58 of the housing56 calls for forming an intermediate molding including a cylindricalarm-to-be portion which has nearly the same dimension as that of the armportion 58 and integrally projects from the cylindrical outer surface ofthe body portion 55 and forming the aforementioned internal thread 57 inthe cylindrical inner surface of the arm-to-be portion of theintermediate molding by inserting a tap 61 into the arm-to-be portionwhile rotating said tap 61. Said tap 61 is provided with an externalthread 60 formed along the cylindrical outer surface of the tap 61.

However, the structure shown in FIG. 7, wherein an internal thread 57 isformed by threading the arm-to-be portion of the intermediate moldingwith a tap 61 that has an external thread 60, enables the efficientprocessing, because it requires a considerably long machining time toperform reliable and stable formation of an internal thread 57 having agiven pitch. Furthermore, great frictional force is generated betweenthe tap 61 and the arm-to-be portion during threading, which frictionalforce wears the tap 61 and reduces its life span.

In order to solve the above problems, an object of the present inventionis to provide a method of producing a ball joint and its housing, ofwhich arm portion that has stable characteristics and is provided withan internal thread formed in its cylindrical inner surface can easilyand quickly be formed.

DISCLOSURE OF THE INVENTION

The present invention provides a method of producing a ball jointhousing which has an arm portion projected therefrom and is adapted tosupport a ball stud, said method calling for inserting a tap that has anexternal thread formed along its cylindrical outer surface into a holeformed in an intermediate molding, said hole having a cylindrical innersurface with a diameter greater than the diameter of said tap, andforming an internal thread in the cylindrical inner surface of said holeby narrowing said intermediate molding, in which the tap is inserted, bycold forging. Compared with a method that calls for forming an internalthread by threading with a tap having an external thread, the method ofthe invention enables the easier and quicker formation with an increasedefficiency. As the invention also prevents wear or other damage to thetap, it is free from the problem of a damaged tap impairing formation ofan internal thread having given dimensions, and therefore facilitatesformation of the arm portion that has stable characteristics.

A method of producing a housing of a ball joint according to anotherfeature of the present invention is characterized in that a pair ofpressure dies respectively provided with concave faces, which are formedat locations opposite each other and are in the shape of a half cylindercorresponding to the half circle of the outer surface of the arm portionwith a curvature corresponding to that of the cylindrical outer surfaceof the arm portion, function to narrow said intermediate molding bysandwiching the intermediate molding between said concave surfaces and,in this state, applying pressure to the intermediate molding. Thisfeature enables the easy formation of an arm portion having stablecharacteristics without deviation of the center axis.

A method of producing a housing of a ball joint according to yet anotherfeature of the present invention calls for forming, prior to narrowingthe intermediate molding, a pair of flat faces cylindrical outer surfaceof the portion of the intermediate molding in which said hole is formed,said flat faces formed in such a manner as to diametrically oppose eachother and extend either in parallel with or at an angle from, exceptperpendicularly, the directions in which the pressure is applied by saidpressure dies. As a result, the method is effective in preventingformation of a burr or other similar defects, which would otherwise beformed on the cylindrical surface of the outer rim of the hole of theintermediate molding by flow of the surplus material in the course ofnarrowing process performed by the pressure dies. This feature thuseliminates the need of cutting work or other work to remove a burr and,consequently, prevent a change of the volume of the housing that wouldotherwise be caused by removal of a burr. Therefore, the inventionensures the easy and reliable formation of a housing having stablecharacteristics.

A method of producing a housing of a ball joint according to yet anotherfeature of the present invention is characterized in that theintermediate molding is formed of an aluminum alloy. Because of thisfeature, the invention facilitates the narrowing of the intermediatemolding in the state where the tap is inserted in the hole. Therefore,the invention is in improving the manufacturability of the housing byensuring the easy formation of a housing, even if the housing will beused as a component that will have to bear a heavy load, such as acomponent of a vehicle.

A ball joint according to the present invention includes a housingformed by the method of producing a housing of a ball joint describedabove so as to have an opening and an inner chamber that directlycommunicates with the opening, a ball stud having a ball head portionand a stud portion positioned such that the ball head potion iscontained in the inner chamber of the housing and that the stud portionis formed as a contiguous, integral body with the ball head portion andprojected from said opening of the housing, and a bearing seat having aninsertion hole and contained in the inner chamber of the housing so thatthe ball head portion of the ball stud is slidably enveloped in thebearing seat and that said stud portion projects from the insertionhole. As a result of using a housing that has efficiently and easilybeen formed with stable characteristics by the method of producing ahousing of a ball joint described above, the manufacturability of theball joint is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway side view of a ball joint according to anembodiment of the present invention; FIG. 2 is a partially cutaway sideview of an intermediate molding used for producing said ball joint; FIG.3 is a side view of an arm-to-be portion of said intermediate moldingused for producing the ball joint; FIG. 4 is a schematic illustration toexplain how a tap is inserted in the arm-to-be portion in the process ofcold forging of the intermediate molding for producing said ball joint;FIG. 5 is a schematic illustration to explain how the arm-to-be portionis narrowed in the process of cold forging of the intermediate moldingfor producing said ball joint; FIG. 6 is a sectional view taken alongthe line A—A of FIG. 4; and FIG. 7 is a schematic illustration toexplain how a housing of a conventional ball joint is formed.

PREFERRED EMBODIMENT OF THE INVENTION

Next, the structure of a ball joint according to an embodiment of thepresent invention is explained hereunder, referring to relevantdrawings.

Referring to FIG. 1, numeral 1 denotes a ball joint, which may beincluded in, for example, the steering mechanism or the suspensionmechanism of an automobile or, provided at an end of a rack shaft of arack-and-pinion steering system.

The ball joint 1 comprises a housing 2 having a generally cylindricalshape with a bottom, a metal ball stud 3, a bearing seat 4 formed ofsynthetic resin, and a dust cover (not shown) which is formed of rubberor a soft synthetic resin into a generally cylindrical shape. Thehousing 2 is formed of an aluminum alloy, such as A6061 (JapanIndustrial Standard: JIS-H-4000), which is an alloy of aluminum,magnesium and silicon.

The housing 2 includes a socket portion 2 a formed in a cylindricalshape with a bottom. The socket portion 2 a has a body portion 13, and adisk-shaped bottom face portion 14. The body portion 13 is open at bothends, i.e. an opening 11 at one end and an insertion hole 12 at theother end. The bottom face portion 14 is integrally attached to the bodyportion 13 in such a manner as to close off the insertion hole 12. Thehousing 2 also includes a cylindrical arm portion 15 that radiallyprojects from the cylindrical outer surface of the body portion 13 as anintegral body with the body portion 13.

The body portion 13 has a cylindrical cover attaching portion 16 whichhas a diameter smaller than the body portion 13 and is coaxially formedalong the rim of the opening 11 as an integral body with the bodyportion 13. A contact portion 17 is formed in the cylindrical innersurface of the body portion 13, at a location close to the insertionhole 12, so that that part of the body portion 13 is indented like astep with its inner diameter increased to nearly the same as the outerdiameter of the bottom face portion 14. A stopper portion 18 adapted tocatch the rim of the bottom face portion 14 so as to hold it and preventits unintentional displacement is formed along the rim of the insertionhole 12 of the body portion 13.

The arm portion 15 is integrally provided at its end with a projectingconnector portion 20, which is formed in a cylindrical shape having adiameter smaller than the arm portion 15 and extends coaxiallytherewith. An internal thread 19 is formed in the cylindrical innersurface of the projecting connector portion 20.

The bottom face portion 14 is inserted into the body portion 13 from theinsertion hole 12 of the body portion 13 until the rim of the bottomface portion 14 comes into contact with the contact portion 17, and thestopper portion 18 is crimped inward. Thus, the housing 2 having agenerally cylindrical shape with a bottom is assembled. The interior ofthe housing 2 assembled as above serves as an inner chamber 22.

The ball stud 3 is comprised of a spherical ball head portion 25, a studportion 27 and a small diameter portion 26, all of which are formed inan integral, contiguous body with the small diameter portion 26 locatedbetween the ball head portion 25 and the stud portion 27. An externalthreaded portion 28 is formed at the end of the stud portion 27.

The bearing seat 4 has a cylindrical portion 31 and a bottom portion 32.The cylindrical portion 31 is formed of a synthetic resin, such aspolyacetal resin or polyurethane resin, that has good bearingcharacteristics and a great ability to withstand load, as well assufficient rigidity and elasticity. The bottom portion 32 is formed ofthe same material as the cylindrical portion 31 into a ring shape andadapted to catch the lower axial end of the cylindrical portion 31. Aninsertion hole 33 adapted to permit the ball head portion 25 of the ballstud 3 to be inserted therefrom into the bearing seat so that the studportion 27 projects from the insertion hole 33 is formed at one of theaxial ends, i.e. the upper end, of the cylindrical body portion 32. Aseating portion 34, which is in a flange-like shape rising inward, isformed around the inner rim of the other end of the cylindrical portion31, i.e. the lower end, so that the bottom portion 32 is permitted toabut and rest on the seating portion 34. The bottom portion 32 is formedin an annular shape so that the curved inner surface of the bottomportion 32 envelopes a part of the ball head portion 25 of the ball stud3, with the outer rim part of the bottom portion 32 overlapping theseating portion 34 of the cylindrical portion 31 and the inner rim partof the bottom portion 32 resting on the bottom face portion 14 of thehousing 2.

The bearing seat 4 is formed by assembling the cylindrical portion 31and the bottom portion 32 into a generally cylindrical shape with abottom. The bearing seat 4 is then placed in the inner chamber 22 of thehousing 2 in the state where the ball head portion 25 is enveloped inthe bearing seat 4. In this state, the center of the ball head portion25 is located at some point along the center axis of the arm portion.The dust cover is provided at one end thereof with a first fittingportion (not shown), which has a ring-like shape and is adapted to fitto the cover attaching portion 16 of the housing 2. At the other endthereof, the dust cover is provided with a second fitting portion whichhas a ring-like shape and is adapted to fit to the stud portion 27 ofthe ball stud 3.

Although not shown in the drawings, a rack-end ball joint having anexternal thread formed along its outer surface or other similar memberis screwed in the projecting connector portion 20 of the arm portion 15of the housing 2 of the ball joint 1.

Next, the process of producing the housing of the ball joint of theembodiment described above is explained hereunder, referring to therelevant drawings.

First of all, by extrusion, pressing, forging, casting or any otherappropriate processing, an intermediate molding 41 shown in FIGS. 2 and3 is formed from a material (not shown) made of an aluminum alloy, suchas A6061 (JIS-H-4000), which is an alloy of aluminum, magnesium andsilicon.

The intermediate molding 41 has a generally cylindrical arm-to-beportion 42 projecting from the cylindrical outer surface of the bodyportion 13, which has an opening 11 at one end and an insertion hole 12at the other end. The arm-to-be portion 42 is formed in a generallycylindrical shape with a given thickness and has a hole 42 a with adiameter greater than the inner diameter of the projecting connectorportion 20 of the arm portion 15. The arm-to-be portion 42 projects froma given location of the body portion 13 in an integral body therewith insuch a manner that the axis of the arm-to-be portion 42 extendsapproximately parallel to the radius of the body portion 13. A pair offlat face portions 42 b respectively having flat faces thatdiametrically oppose each other and extend parallel to the axis of thebody portion 13 are formed on the cylindrical outer surface of the bodyportion 13.

The bottom face portion 14 is integrally attached to the body portion 13in such a manner as to close off the insertion hole 12. The housing 2also includes a cylindrical arm portion 15 that radially projects fromthe cylindrical outer surface of the body portion 13 as an integral bodywith the body portion 13.

As shown in FIG. 4, a tap 44 that has an external thread 43 formed alongits cylindrical outer surface is inserted into the hole 42 a, whichdefines the cylindrical inner surface of the arm-to-be portion 42 of theintermediate molding 41. The external thread 43 of the tap 44 has thesame pitch as that of the external thread of the rack-end ball joint ofthe tie rod or other similar member to be connected to the ball joint 1.

Thereafter, the arm-to-be portion 42 is set in a pressing machine 46 inthe state where the tap 44 is in the hole 42 a of the arm-to-be portion42. As shown in FIGS. 5 and 6, the pressing machine 46 has a pair ofpressure dies, i.e. an upper die 47 and a lower die 48. The opposingsurfaces of the upper die 47 and the lower die 48 are respectivelyprovided with concave faces 49,49. Each concave face 49 is formed in aconcave surface in the shape of a half cylinder approximatelycorresponding to the half circle of the outer surface of the arm portion15, with a curvature corresponding to that of the cylindrical outersurface of the projecting connector portion 20 of the arm portion 15.When the upper die 47 and the lower die 48 are brought into contact witheach other, the concave faces 49,49 together define a cylindrical shapehaving a diameter equal to the outer diameter of the projectingconnector portion 20 of the arm portion 15.

The arm-to-be portion 42, in which the tap 44 is inserted, is positionedbetween the two opposing concave faces 49,49 of the upper and lower dies47,48 of the pressing machine 46. Thereafter, by moving the upper andlower dies 47,48 towards each other, in other words approximately alongthe flat face portions 42 b, pressure is applied to the arm-to-beportion 42 in such a direction as to flatten the arm-to-be portion 42,which is sandwiched between the upper die 47 and the lower die 48, sothat the diameter of the arm-to-be portion 42 is reduced with thesurplus material being forced to move towards the flat face portions 42b. As a result of the reduction of the diameter of the arm-to-be portion42, the external thread 43 of the tap 44 digs into the cylindrical innersurface of the arm-to-be portion 42 so that an internal thread 19 havinga pitch corresponding to the external thread 43 is formed in thecylindrical inner surface of the hole 42 a of the arm-to-be portion 42.Thus, an arm portion 15 provided with a projecting connector portion 20that has given outer and inner diameters and a given thickness isformed. Thereafter, with the tap 44 being pulled out from the arm-to-beportion 42 while being rotated, formation of a housing 2 that includesan arm portion 15 having a projecting connector portion 20 is completed.

According to the embodiment described above, an arm portion 15 having aprojecting.connector portion 20 is formed by following a procedurecomprising inserting the tap 44, which has an external thread 43 formedalong its cylindrical outer surface, into the hole 42 a of thecylindrical arm-to-be portion 42 of the intermediate molding 41, whicharm-to-be portion 42 integrally projects from the cylindrical outersurface of the body portion 13 and has an inner diameter greater thanthe inner diameter of the projecting connector portion 20 of the armportion 15, and, thereafter, reducing the diameter of the arm-to-beportion 42 by cold forging so as to form an internal thread 19 in thecylindrical inner surface of the hole 42 a. Therefore, by press working,which is a kind of cold forging, the process according to the embodimentof the invention enables the easy and quick formation of an arm portion15, thereby increasing the forming efficiency, compared with such aconventional method that calls for forming an internal thread 19 byusing a tap 44 having an external thread 43 to thread the cylindricalinner surface of an arm-to-be portion 42, which has been so formedbeforehand as to have the same dimensions as the arm portion 15. Anotherbenefit of the embodiment described above lies in that it prevents wearor other damage to the tap 44; it is free from the problem of a worn tap44 impairing formation of an internal thread 19 having a given depth andis therefore effective in increasing the yield of the housing 2.Furthermore, by the increased life span of the tap 44, the embodimentenables the easy and efficient production of the arm portion 15 havingstable characteristics. These benefits are consequently effective inimproving the manufacturability and reducing the production cost of aball joint 1 that uses a housing 2 easily and efficiently formed withstable characteristics by the production method according to theembodiment described above.

By using a pressing machine 46 having a pair of dies, i.e. an upper die47 and a lower die 48, respectively having concave faces 49,49 thatoppose each other, the narrowing of the arm-to-be portion 42 isperformed in such a manner that the arm-to-be portion 42 is sandwichedby the concave faces 49,49, each of which is in the shape of a halfcylinder approximately corresponding to the half circle of the outersurface of the arm portion 15 and having a curvature corresponding tothat of the cylindrical outer surface of the projecting connectorportion 20 of the arm portion 15, and that pressure is applied to thearm-to-be portion 42 in such a direction as to flatten the arm-to-beportion 42. According to the method described above, the center axis ofthe arm-to-be portion 42 does not deviate from the center axis of theprojecting connector portion 20 of the arm portion 15. Therefore, themethod enables the easy formation of an arm portion 15 that is providedwith a projecting connector portion 20 having stable characteristicswith the minimal possibility of deviation of the center axis.

Furthermore, the arm-to-be portion 42 of the intermediate molding 41 hasflat face portions 42 b,42 b diametrically opposing each other. The pairof dies, i.e. the upper die 47 and the lower die 48 which face towardeach other, reduce the diameter of the arm-to-be portion 42 bysandwiching the arm-to-be portion 42 and applying pressure thereto inthe direction in which the flat faces of the flat face portions 42 b,42b extend. As a result, the surplus material is forced to move into thespace defined by each flat face portion 42 b and the corresponding partsof the concave faces 49,49 of the upper and lower dies 47,48. Therefore,the method of the embodiment is effective in preventing formation of aburr that might otherwise be caused by the surplus material flowing intothe gap between the upper and lower dies 47,48 and impair uniformapplication of a given pressure and consequently inhibit the arm portion15 from having given, appropriate dimensions. The embodiment thusenables the easy and reliable formation of a housing 2 that includes anarm portion 15 having stable characteristics without requiringcomplicated cutting work for removing a burr.

The material used by the present embodiment is an aluminum alloy, suchas A6061 (JIS-H-4000), which is an alloy of aluminum, magnesium andsilicon. Because of this feature, the embodiment improves themanufacturability of the housing by enabling the easy formation of ahousing 2 that includes an arm portion 15 having an internal thread 19and is ensured of stable characteristics for a long period of timewithout fluctuation of the characteristics, even if the housing 2 is apart of a ball joint 1 used in a vehicle or as any other component thatwill have to bear a heavy load. Yet another benefit of the embodimentlies in that the internal thread 19 can easily be formed, because analuminum alloy is a soft alloy.

The usage of the embodiment described above is not limited to thesteering mechanism or the suspension mechanism of an automobile; theembodiment is applicable to any kind of ball joint, including thoseprovided with no dust covers or having bearing seats in other shapes.

The invention is explained as above referring to the embodiment, whereinan arm portion 15 that is provided at its end with a projectingconnector portion 20 having an internal thread 19 is formed by pressinga part of the arm-to-be portion 42 so as to narrow that part. However,it is also possible to form the entire arm portion 15 into a projectingconnector portion 20 having a uniform outer diameter by pressing nearlythe entire cylindrical part of the arm-to-be portion 42.

Furthermore, the method of narrowing process is not limited to pressworking, and any method (for example, rolling a roller along thecylindrical outer surface of the arm-to-be portion 42) may be employed,as long as narrowing of the arm-to-be portion 42 is performed in thestate where a tap 44 is inserted in an arm-to-be portion 42. There isthe possibility of the axial dimension of the arm-to-be portion 42becoming excessively long due to flow of the surplus material during thenarrowing of the arm-to-be portion 42. Depending on the manner ofnarrowing process, however, such a problem can be solved by setting thelength of the arm-to-be portion 42 beforehand so that the arm portionhas a given length when its formation is completed. Thus setting thelength of the arm-to-be portion 42 eliminates the necessity of cuttingor other post-formation processing of the end of the arm-to-be portion42, and consequently improves the efficiency in formation.

According to the embodiment described above, the pressing machine 46 hasa pair of dies, i.e. an upper die 47 and a lower die 48, and performsnarrowing of the arm-to-be portion 42 by sandwiching it between theupper die 47 and the lower die 48. However, the pressing machine 46 mayhave any other appropriate structure, examples of which include onehaving a split mold divided into a plurality of radially arranged diesthat can be moved either actively, or in relation to moving thearm-to-be portion 42, in the direction of the center of the split moldso as to reduce the inner diameter of the split mold.

Furthermore, although the above explanation involves a structureprovided with a single arm portion 15, the explanation is alsoapplicable to any other structures having a plurality of arm portions.

It is possible to perform narrowing of the arm-to-be portion 42 of theintermediate molding 41 after the ball stud 3 or the like is attached tothe body portion 13 of the intermediate molding 41.

The ball stud 3 does not always have to be positioned such that thecenter of the ball head portion 25 of the ball stud 3 contained in thehousing 2 is located at some point along the center axis of the armportion 15; for example, the center axis of the arm portion 15 may bepositioned at a slight distance from the center of the ball head portion25 of the ball stud 3, or the arm portion 15 may project from thecylindrical surface of the body portion 13 at an angle therefrom.However, positioning the ball stud 3 such that the center of the ballhead portion 25 of the ball stud 3 contained in the housing 2 is locatedon the center axis of the arm portion 15 in the same manner as theembodiment described above is effective in increasing the bucklingstrength of the ball joint 1 with respect to load applied to the balljoint.

Although the intermediate molding 41 is formed of an aluminum alloy,such as A6061 (JIS-H-4000), which is an alloy of aluminum, magnesium andsilicon, according to the embodiment, any material may be used providedthat its shape can be changed so as to be narrowed and permit aninternal thread 19 to be formed in its inner surface. Althoughexplanation has been given referring to the arm-to-be portion 42 havingflat face portions 42 b,42 b, it is also possible to form the flat faceportions 42 b,42 b by press working or cutting work after formation ofan intermediate molding 41 that has a cylindrical arm-to-be portion 42.However, it is not essential to provide flat face portions 42 b,42 b.

POSSIBLE INDUSTRIAL APPLICATION

As described above, a ball joint according to the present invention istypically used in the state where it is attached to an end of the rackbar of the rack-and-pinion steering system, the steering mechanism orthe suspension mechanism of an automobile.

What is claimed is:
 1. A method of producing a housing of a ball joint,the housing including an arm portion projecting therefrom and beingadapted to support a ball stud, the method comprising: inserting a tapthat has an external thread formed along a cylindrical outer surfacethereof into a hole formed in an intermediate molding, the holecomprising a cylindrical inner surface having a diameter greater than adiameter of the tap, and forming an internal thread in the cylindricalinner surface by narrowing the diameter of the cylindrical inner surfaceof the hole containing the tap therein by cold forging, wherein theforming of the internal thread comprises providing a pair of pressuredies respectively having concave faces, formed at locations facing eachother when the pressure dies are combined for the cold forging and eachhaving a shape of a half cylinder corresponding to a half of acylindrical outer surface of the arm portion that is split along thelongitudinal central axis of the arm portion, the curvature of theconcave faces being substantially equal to the curvature of thecylindrical outer surface of the arm portion, placing the intermediatemolding between the concave faces and applying a pressure to theintermediate molding through the pressure dies containing theintermediate molding therein.
 2. The method of producing a housing of aball joint of claim 1, wherein further comprising, prior to the formingof the internal thread, forming a pair of flat faces on a cylindricalouter surface of the intermediate molding at locations corresponding tothe application of the pressure, the flat faces being diametricallyopposite from each other and extending either in parallel with or at anangle from, except a right angle, the direction in which the pressure isapplied to the intermediate molding by the pressure dies.
 3. The methodof producing a housing of a ball joint of claims 1 or 2, wherein theintermediate molding is made of an aluminum alloy.
 4. A method ofproducing a housing of a ball joint, the housing including an armportion projecting therefrom, the method comprising: providing acylindrical intermediate molding that has a cylindrical internal holetherein; removing a first portion of the cylindrical intermediatemolding along the cylindrical internal hole; inserting a tap that has anexternal thread formed along a cylindrical outer surface thereof intothe cylindrical internal hole, the diameter of the cylindrical internalhole being greater than the diameter of the tap; and applying a pressureto a portion of the intermediate molding that includes the portion fromwhich the first portion is removed so that the external thread of thetap is impressed on a surface of the internal cylindrical hole of theintermediate molding.
 5. The method of producing a housing of a balljoint of claim 4, further comprising removing a second portion of thecylindrical intermediate molding along the cylindrical internal holeprior to the application of the pressure, wherein the first and secondportions are located opposite from each other with respect to thecentral axis of the cylindrical internal hole.